1. Field of the Invention
The present invention relates to a variable displacement pump. The pump may be used, for example, in a hydraulic power steering apparatus for a motor vehicle or the like.
2. Description of the Related Art
A variable displacement pump used in a hydraulic power steering apparatus for a motor vehicle is driven by an engine of the motor vehicle, and a discharge flow rate is increased in accordance that a rotating speed of the engine or a rotating speed of the pump becomes higher. On the contrary, in the hydraulic power steering apparatus for the motor vehicle, a high assist force is necessary during stopping or a low speed traveling region, and lower assist force is desirable in a high speed traveling region, to improve a steering property and driving stability.
A variable displacement pump is described in Japanese Patent No. 3501990 (patent document 1). A rotor is fixed to a pump shaft inserted to a pump casing so as to be rotationally driven, and accommodating a plurality of vanes in a groove so as to be movable in a radial direction, a cam ring fitted to a fitting hole within the pump casing so as to be movable and displaceable, forming a pump chamber with respect to an outer peripheral portion of the rotor and forming first and second hydraulic chambers with respect to the pump casing, and a discharge flow rate control apparatus selectively introducing a pressure in an upstream side of a main throttle provided in a pump discharge side passage and a pressure in a pump suction side to the first hydraulic chamber, and selectively introducing a pressure in a downstream side of the main throttle and a pressure in the pump suction side to the second hydraulic chamber.
In a low speed traveling region in which the rotating speed of the pump is low, a switch valve of the discharge flow rate control apparatus is positioned at an original position so as to connect the first hydraulic chamber to the pump suction side and connect the second hydraulic chamber to the downstream side of the main throttle in the pump discharge side. Accordingly, the cam ring is maintained in a side in which a volumetric capacity of the pump chamber is maximized on the basis of a pressure difference between the first hydraulic chamber and the second hydraulic chamber, and the pump discharge flow rate is increased in proportion to the rotating speed.
If the rotating speed of the pump is increased, the switch valve of the discharge flow rate control apparatus is moved so as to connect the upstream side of the main throttle to the first hydraulic chamber, and connect the second hydraulic chamber to the pump suction side. Accordingly, the cam ring is moved to the side in which the volumetric capacity of the pump chamber is made smaller, and a pump discharge flow rate V cancels a flow rate increase V1 caused by an increase of the rotating speed and a flow rate decrease V2 caused by a volumetric capacity reduction of the pump chamber, with respect to the increase of the rotating speed, thereby intending to maintain a fixed flow rate.
In the variable displacement pump described in the patent document 1, in the case that the cam ring reaches the moving end side in which the volumetric capacity of the pump chamber is minimized, in the high rotation region of the pump, the flow rate decrease V2 caused by the volumetric capacity reduction of the pump chamber hits the peak. Accordingly, the discharge flow rate V of the pump obtains only the flow rate increase V1 caused by the increase of the rotating speed and is increased. Therefore, an assist force in the high speed driving region becomes higher, and a steering feeling is rapidly light so as to become unstable.